A simple routine blood test (red cell distribution width, RDW) is a sensitive detector of iron deficiency at a state earlier than previously has been easily detectable. With this test iron deficiency can be detected before there is anemia, and even before the mean red cell size (mean cell volume, MCV) becomes abnormal. This blood test now is part of the standard routine "complete blood count" performed as part of the data base in 2,000 hospitals in this country. An initial pilot study suggests that screening with the present standard screening method, stool guaiac. However, the RDW detected 3 of 3 colorectal carcinomas; the stool guaiac, only 2 of 3 in the same population. The present hypothesis is that screening populations at risk for colorectal carcinoma will be easier, more sensitive, and better accepted by the patient with this routine blood test than with the method currently used, testing for occult blood in the stool. To test this hypothesis the following will be done: 1. Define three sample groups, to total approximately 10,000 subjects, at risk for colorectal carcinoma: a large group of patients at the University of Texas Medical Branch, and two smaller groups of representative outpatients. 2. Develop an algorithm to identify normal and potentially abnormal subjects, and then in the latter group confirm or rule out the presence of disease. The algorithm will include as screening procedures both the present method (occult blood in the stool) and the proposed method (RDW). 3. Apply this algorithm for screening to the populations at risk. 4. Analyze the results of the study to determine: a. Subject acceptance of the two tests. b. Comparative sensitivity and specificity of the two tests to detect colorectal carcinoma. c. Cost of the two procedures. 5. Conclude whether fecal occult blood testing, RDW, or a combination of the two should be the preferred method of screening populations at risk for colorectal carcinoma.